The present invention relates to a developer used for developing a presensitized plate ("water-less PS plate") for use in making a lithographic printing plate which has a silicone rubber layer as an ink repellent layer and requires no dampening water during printing ("water-less lithographic printing plate"). More specifically, the present invention relates to a developer for a water-less PS plate which is excellent in safety, in developing ability and in half-tone dots reproduction.
There have already been proposed various kinds of water-less PS plates for lithographic printing plates having a silicone rubber layer as an ink repellent layer. In particular, those disclosed in, for instance, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J.P. KOKOKU") Nos. Sho 54-26923 and Sho 56-23150 comprise, on a substrate, a photopolymerizable adhesive layer and a silicone rubber layer in this order. This water-less PS plate is imagewise exposed to actinic rays through a positive film which is in close contact with the PS plate under vacuum, only the silicone rubber layer on the unexposed portions of the exposed water-less PS plate is selectively peeled off by dipping the exposed plate in a developer consisting of or mainly comprising a parafinic hydrocarbon to expose the unexposed photopolymerizable adhesive layer below it and to thus form ink-receptive image portions. On the other hand, the silicone rubber layer on the exposed portions gets swollen with the developer, but is strongly adhered to the photopolymerizable adhesive layer and, therefore, remains on the surface of the plate. The latter portions serve as ink-repellent non-image portions. In this way, a water-less lithographic printing plate is formed.
The water-less lithographic printing plate obtained through such a plate-making process often exhibits low fine half-tone dots reproduction when it is incompletely developed, because a part of the silicone rubber layer remains even on the portions from which it should completely be peeled off or removed. Moreover, when the plate surface is rubbed excessively strongly, even the silicone rubber layer, which must be positioned on the surface of the printing plate and serves as the non-image portions, is damaged.
Under such circumstances, to improve the developability of a water-less PS plate, there is proposed a method in which a polar solvent capable of dissolving even the photopolymerizable adhesive layer is added to the developer to dissolve a part of the light-sensitive layer while the silicone rubber layer is removed by rubbing the plate surface. However, the light-sensitive layer of the non-image portions also gets swollen to some extent as the amount of the polar solvent increases and, as a result, the silicone rubber layer on the non-image portions is also rubbed off. On the other hand, if the amount of the polar solvent is insufficient, a satisfactory effect to be attained by the addition cannot be expected.
J.P. KOKOKU No. Sho 63-21890 (=U.S. Pat. No. 4,378,423) proposes the use of a specific propylene oxide compound among polar solvents, which improves the developing ability of the developers. In addition, the resulting developers do not dissolve out the light-sensitive layer of the image portions and do not swell the light-sensitive layer of the non-image portions even if the amount of the propylene oxide compound is increased. Therefore, the developers can provide a lithographic printing plate wherein the shadowed portions comprising fine half-tone dots of the silicone rubber are sufficiently reproduced.
However, the developer consists of 100% organic solvents and, therefore, they suffer from a problem of safety because it may easily catch fire.
Under such circumstances, the present inventors have made intensive studies for providing a developer for water-less PS plates having a silicone rubber layer as an ink repellent layer, which developer does not ignite under the usual conditions by solubilizing it in water, and have tried to prepare a developer for water-less PS plates mainly comprising water to which the foregoing propylene oxide compound is added according to the methods disclosed in Japanese Patent Unexamined Publication (hereunder referred to as "J.P. KOKAI") No. Hei 1-159644 (EP0320945A) and J.P. KOKAI No. Sho 61-275759. However, the inventors failed to develop such a developer because the propylene oxide compound is highly soluble in water.